The null reference glide path system, which forms one element of the standard Instrument Landing System (I.L.S.), relies on ground reflection to obtain the appropriate antenna radiation patterns. As the ideal system requires an infinite plane reflector, significant irregularities in the glide path are observed for actual sites with non-flat ground. At present, objectionable irregularities cannot be predicted in detail for an arbitrarily specified site.
A satisfactory solution to this problem would require:
(i) an accurate method of measuring the glide path at any given installation;
(ii) an economical approach to the modification of existing sites to meet the glide path specifications for any desired performance category;
(iii) a systematic method of synthesis of a suitable ground surface, or the adoption of alternative systems which are independent of ground effects.
The terrain problem can be investigated in accordance with (i), (ii) and (iii) above by means of geometrical modelling. A microwave model has been developed, for which the primary requirements were:
(i) a measuring facility to permit the calculation of Difference in Depth of Modulation (D.D.M,) to an accuracy of approximately 3% (i.e. 0.01◦ in elevation angle);
(ii) a contoured surface-modelling technique to maintain an accuracy of 3 in. (full size) in ground height.
These tolerances were adopted to provide resolution of path bends and changes in sensitivity consistent with the performance specifications for International Civil Aviation Organization (I.C.A.O.) Category III installations.
The utility of the model approach was tested by modelling the Avalon (Victoria) aerodrome site. Several significant differences between the flight test and model results were obtained, although the measured quality of the glide path in each case was similar. The model results established the significance of a ground region between 3700 ft. and 4200 ft. from the transmitting antennas. Several possible methods of improving the Avalon site are considered.